1. Field of the Invention
The present invention relates to wireless communication systems; and more particularly to detecting collisions between data packets in wireless communication channels which rely on a collision detection protocol.
2. Description of Related Art
One common network protocol is based on the so-called carrier sense, multiple access with collision detection technique (CSMA/CD). This technique is used, for instance, in wired Ethernet standard LANs which are in widespread use. However, this protocol has not been successfully applied to wireless channels.
In the wireless environment, a transmitting signal from a station will have a level much higher than the weakest received signal that might be valid within a basic service area of transmission. For instance, in one application, the transmitting signal is issued at 0 dBm, while the weakest received signal, which is expected to be valid, will be detected at a level of -75 dBm, resulting in a ratio of 31.6 million to 1 in power. During transmission of a signal, detecting the presence of a signal millions of times smaller in magnitude is quite difficult.
The CSMA/CD protocol operates under several fundamental assumptions
a. Control of access to the media is distributed among all users on the network.
b. Medium behavior is well defined (deterministic) and constant (not stochastic).
c. Equal and full connectivity.
d. Listen before talk (Carrier Sense part).
e. Once data transmission is initiated, if a collision is detected, all parties stop transmitting and go into a random back off mode before retransmitting.
The highest utilization of the bandwidth, with the lowest protocol overhead, is maintained because once the collision is detected, it is recoverable without wasting network bandwidth by allowing all the colliding parties to transmit to completion.
The CSMA/CD protocol has one other fascinating attribute. It scales well under heavy load. For instance, the higher the demand for bandwidth (due to large number of users, or high traffic requirements), the higher the probability will be of packet collisions. The IEEE 802.3 (Ethernet) protocol does not exhibit extreme performance degradation under these conditions; other protocols do.
In a well tuned environment, efficiency numbers above 90% are obtainable. Without collision detection (the CSMA/CA--Collision Avoidance situation), much lower efficiency numbers ranging between 18% and 30% are obtainable because bandwidth is wasted as collisions are allowed to run to completion.
Most wireless developers believed that true wireless collision detection was not obtainable. This belief arises because the mechanism used to detect a collision on a wire (i.e., looking for a dc offset on the wire) does not translate to the wireless environment.
To compensate for this problem, several alternative medium access control (MAC) protocols have been developed. They all share the fundamental characteristic of a "reservation." This technique attempts to reserve either bandwidth (time or frequency), or hardware, for each individual user, to guarantee collisions do not occur.
The following is a representative, but not inclusive, list of common MAC protocols to compensate for the differences between the wired and wireless medium, they are:
a. Time Division Multiple Access
b. Code Division Multiple Access
c. Frequency Division Multiple Access
d. Reservation Based Systems such as Request to Send/Clear to Send/Acknowledgment (RTS/CTS/Ack).
The problems with these techniques are that they require one central Point Coordination Function (PCF) to handle "reservation" requirements (a, b, and c), or some form of collision period (d). The result is a very high protocol overhead. For instance, prior art systems burn roughly 50% to 60% of bandwidth with protocol overhead, in some cases. In addition, the overhead problem is complicated under heavy load conditions.
Accordingly, it is desirable to provide a technique for true collision detection in a wireless environment, to enable more efficient protocols, such as CSMA/CD.